ImageReader: Set BufferItem mGraphicBuffer to null when returning it

The ImageReader JNI layer needs to maintain a list of BufferItems that
back the Java Image objects. At start, we allocate maxImages of these
in a list, and acquire and release them from this list as the
application acquires and releases Images.

However, on release, the contents of the BufferItem are not adjusted,
so it can retain a reference to a GraphicBuffer until the BufferItem
is reused.

Test: Camera CTS passes, memory usage drops after camera bursts
Bug: 62652080
Change-Id: I9aade5e32cbe0af0c15a3886e16ada1a70392b74